The present invention relates, in general, to process control systems and, in particular, to a new and useful system for controlling the blending of two substances, in particular a concentrated fluid with a diluting fluid.
In many processes, the quality control of a product is supplemented by a blending operation which comprises the mixing of two or more fluid streams to achieve a desired product property. For example, the mixing operation is utilized to control concentrations or specific gravity of a product.
Blending control systems are used which are based upon maintaining the flow rate of the two feed streams at a ratio. This ratio is used to determine a flow rate of the diluting stream from a given flow rate of the concentrated or rich stream. The drawbacks of this arrangement are that the ratio of the two feed streams is normally determined on the basis of design conditions. Such design conditions include temperature, pressure and compositions. The design conditions are assumed for the control system. The actual operating conditions of the system are normally not the same as the design conditions, however. This requires an updating of the ratio to compensate for such changes.
An additional factor which is not considered in the prior art control systems is that, in many blending operations, for example the dilution of hydrochloric acid, sodium hydroxide or potassium hydroxide, the temperature of the product stream changes due to the heat generated (consumed) when the concentrated stream is diluted. The prior art systems do not take the heat of the solution into account. Consequently, the product volume changes due to the change in temperature and thus the product concentration is changed undesirably.
In addition, the individual temperatures of the concentrated and diluting streams may vary. The temperature of water drawn from a city supply, for example, varies with climate. This also contributes to a deviation of the product properties from desired values.
Pertinent prior art references to the present invention include a U.S. Pat. No. 3,294,859 to Prater et al. This patent is related to the analysis and control of multireaction systems. A control system for a blending operation is not disclosed however.
U.S. Pat. No. 3,254,107 to Morgan et al discloses a control system for blending two streams of different substances but does not consider all of the parameters which are considered according to this invention to insure uniform product properties despite fluctuations in the parameters. U.S. Pat. No. 3,751,644 to Meyer also discloses a control system for a blending operation which, similarly, does not consider all of the parameters which may fluctuate to insure uniform product properties.
Other relevant but non-anticipating references are U.S. Pat. No. 3,940,600 to Alexander et al and U.S. Pat. No. 4,272,823 to Pool.